1. Field of the Invention
The present invention relates to a nitride semiconductor laser element and a method for manufacturing the nitride semiconductor laser element.
2. Background Information
Nitride semiconductor laser elements hold promise as light sources for next-generation optical disks, light sources for displays, and light sources for exposures.
A nitride semiconductor substrate is used for growing a semiconductor layer in a nitride semiconductor laser element. A nitride semiconductor substrate has high dislocation density regions and low dislocation density regions.
A nitride semiconductor layer with a laminar structure is obtained by crystal growth of a nitride semiconductor layer by metal-organic chemical vapor deposition (MOCVD), for example.
Low manufacturing yield is generally a problem with nitride semiconductor laser elements, and the substrate is one of the reasons for this.
At present, a GaN substrate is used as a growth substrate for growing a nitride semiconductor layer. A GaN substrate is different from a GaAs substrate or InP substrate in that manufacturing a low-dislocation substrate that is uniform overall in the form of a wafer is difficult. And even if a low-dislocation substrate can be manufactured, a new problem is the increased diameter of the substrate. Accordingly, GaN substrates in which dislocations are included in the wafer are being used at present. When a GaN substrate such as this is used as a substrate for growing a nitride semiconductor layer, dislocations propagate from the high dislocation density regions of the substrate to the nitride semiconductor layer. This poses the risk of adversely affecting the characteristics of the drive (main light emission) region of the nitride semiconductor layer.
Also, since the lattice constants do not match perfectly between the GaN substrate and the nitride semiconductor layer, tensile strain is produced in the nitride semiconductor layer. This tensile strain creates cracks within the wafer, which lower the yield of the product.
A nitride semiconductor laser element in which recesses are formed has been proposed in order to solve these problems (see JP-2004-327879-A).
The nitride semiconductor laser element disclosed in this Patent Document is produced by growing a nitride semiconductor layer on a nitride semiconductor substrate, and then forming recesses in the nitride semiconductor layer. These recesses prevent dislocations (defects) from propagating to the laser waveguide region or the area around the laser waveguide region.
Also, a technique has been proposed in which recesses are formed in high dislocation density regions (regions of concentrated dislocations) and low dislocation density region (regions of non-concentrated dislocations) (see JP-2005-294416-A).
As disclosed in FIGS. 12 and 13 of this Patent Document, this technique involves growing a nitride semiconductor layer on a nitride semiconductor substrate having recesses.
However, with JP-2004-327879-A, since the recesses are formed after the nitride semiconductor layer has been grown on the nitride semiconductor substrate, any dislocations (defects) that occurred at the stage of growing the nitride semiconductor layer, and particularly any dislocations that have propagated to the laser waveguide region, cannot be eliminated. Also, growing the nitride semiconductor layer over a nitride semiconductor substrate that includes dislocations makes tensile strain more likely to occur, so cracks are more prone to develop. Furthermore, tensile strain is produced by etching, growing the nitride semiconductor layer, and so forth in subsequent device steps, and this is another source of new cracks.
With JP-2005-294416-A, merely forming recesses in the high dislocation density region can prevent the reoccurrence of dislocations, but it can not adequately suppress cracking, therefore producing nitride semiconductor laser elements at a stable yield cannot be expected.